Cable television systems are well known. These systems are usually comprised of a headend with one or more trunk lines extending therefrom with each trunk line having a plurality of feeder lines extending therefrom into subscriber areas where each subscriber is attached via a line tap onto the feeder or service line. If the distances between the headend and subscriber areas are substantial, intervening distribution hubs may be located along the trunk lines to replenish the strength and quality of the signal being provided to subscribers. Distribution hubs simply act as small headends and exist to ensure the quality of delivered signal in large CATV networks. Each distribution hub may, in turn, be coupled to a plurality of service sites by feeder lines. Each service site may have one or more service lines extending therefrom to couple a plurality of subscribers to the service site. In this network, a transmission signal is provided over the trunk lines to the distribution hubs or service hubs. This amplified signal is then provided to the feeder lines extending from the distribution hub or service hub to provide the signal to the service sites. If the distance between a distribution hub and service site is so great as to erode signal strength to an unusable level, another distribution hub may be interposed between the service site and first distribution hub to amplify the signal strength again. Amplification occurs along trunk, feeders, and service lines as necessary to maintain the transmission signal at an adequate level before being provided to subscriber equipment. Taps located at each subscriber site bring the transmission signal into a subscriber's site.
The transmission signal from the headend may include entertainment signals and data signals. The entertainment signals may be received as broadcast signals received via satellite from a broadcast signals originating location. At the headend, each broadcast signal is placed on its own channel within the spectrum of the trunk, feeder and service lines used in the CATV system. The spectrum of the lines coupling the CATV system together is the range of frequencies supported by the communication conduits used for the lines. In a typical CATV system, this spectrum is divided into a transmission portion and a return portion. The return portion of the spectrum may be used to support data transmissions, telemetry, and/or control information from subscriber sites back to the headend. The data transmissions from subscribers typically include status information about the subscriber's equipment which may be used by components at the headend to ascertain the status of the cable system or subscriber equipment. The most common types of spectrum splitting methods are called sub-split, mid-split and high split. Sub-split means a lower portion of the spectrum smaller than the transmission spectrum is available for the return spectrum. Mid-split means that the spectrum is allocated one-half to the transmission portion and one-half to the return spectrum. High split means an upper portion of the spectrum smaller than the return spectrum is used for the transmission spectrum.
At the headend, each broadcast signal is allocated to a channel in the transmission spectrum. In a sub-split system, the first channel in the transmission spectrum begins at 55 MHz, for example. The width of the channel varies according to the standard used for the system. In the United States, most CATV systems use National Television System Committee (NTSC) standard which allocates 6 MHz to each channel. In Europe, the Phase Alteration Line (PAL) standard is used which allocates 8 MHz to each channel. The frequency of a broadcast signal may be up-shifted or down-shifted to place the broadcast signal on one of the channels of the transmission portion of the spectrum of the transmission signal provided by the headend. The data signals at the headend may be received from one or more digital data sources (including subscriber equipment) and these signals may also be placed on a channel in the transmission signal for distribution through the network. Typically, display devices such as televisions or the like at the subscriber sites use the broadcast signals to generate audio and video while data devices such as cable modems, or other intelligent devices, convert the data signals for use by computers or the like.
The trunk, feeder and service lines of many CATV systems are all coaxial cables. Because the signals carried by coaxial cables are electrical, these systems are susceptible to electrical and electromagnetic noise from natural phenomena and other electrical or magnetic sources. In an effort to improve the clarity of the signals carried over a CATV system, coaxial cables used for trunk and feeder lines are being replaced by fiber optic cables. Because fiber optic cable carries light signals, the signals are less susceptible to electrical and electromagnetic noise from other sources. Additionally, fiber optic cables carry signals for longer distances without appreciable signal strength loss than coaxial cable. However, the cost of replacing coaxial cable with fiber optic cable has prevented many companies from converting their service lines to fiber optic cable. CATV systems having both fiber optic trunk and feeder lines along with coaxial service lines are typically called hybrid fiber cable (HFC) systems. In HFC systems, the service sites where the light signal from a fiber optic cable is converted to an electrical signal for a coaxial service line is called a fiber node.
Previously known CATV systems have limitations for supporting data communication in the return spectrum of a system. In a typical sub-split CATV system, the return spectrum is in the range of approximately 5 to 42 MHz. This leaves, at best, approximately six (6) channels for data communication back to the headend using the NTSC standard and about four (4) under the PAL standard. However, not all of these channels are equally desirable for data communication. Some of the channels in this range are more susceptible to noise degradation than other channels. As a result there are few good channels for data communications in a sub-split system which is probably the most commonly used system type in the United States. In addition, standards are under development which may define channel widths for forward and return spectrum that are different than NTSC or PAL standards already established.
Even if all the channels in the sub-split range are available for data communication use, other limitations arise as the number of subscribers in the system increase. Allocating the subscribers coupled to a service line to the channels available in a return spectrum may place a reasonable number of subscribers on each channel. At the service site or fiber node, though, all of the service lines are typically merged so all subscribers coupled to the service site or fiber node are allocated to the same available channels in the return spectrum of the cable connecting the service site to the distribution hub. At the distribution hub, the data messages from each service site or fiber node coupled to the distribution hub are merged into the same spectrum of a trunk or feeder line. This merger of data messages from lower network levels to the return spectrum of a single cable continues up to the headend. In an effort to prevent all of the channel capacity being shared by a group of subscribers from being consumed, a time frequency, or other multiplex scheme may be used. While this method allocates a time slot or frequency band on a channel for a subscriber, the time or spectrum available for messages decreases as the number of subscribers decreases. For example, if a fiber node has four lines extending from it with each line having 125 customers, the 500 customers coupled to a service site or fiber node are put on six or fewer channels. At the distribution hub coupled to the fiber node, there may be, for example, three other fiber nodes coupled as well. As a result, 2000 subscribers now contend for data message space on the same six channels. In a large metropolitan area where the number of subscribers may be 200,000 or more, there may be as many as 30,000 subscribers or more per channel. Consequently, message traffic within a channel may become congested and overall performance of the messaging system degraded. Likewise, the response time for messages is significantly increased as each subscriber must contend with a large number of other subscribers for space on a channel within the return spectrum of the system.
What is needed is a way to allocate the available return spectrum in a CATV system to subscribers throughout the network without requiring all of the subscribers to contend for the same channels within the return spectrum of a cable.